The invention relates to a wrapping machine for wrapping a product with a stretch plastic film and wrapping methods for wrapping the film on the product.
Wrapping machines are known comprising a supporting frame, a supporting structure and a reel-holding carriage.
The supporting frame, which is bridge-shaped, is arranged for supporting the supporting structure and is positioned at a zone in which it is desired to wrap the products, conveyed thereto, for example, by a conveyor belt, that is slidable under the supporting frame.
The supporting frame further comprises a carriage arranged for moving the supporting structure along a vertical axis.
The supporting structure comprises a rotating ring rotated around a vertical wrapping axis by a belt driven by a gear box.
The rotating ring supports the reel-holding carriage and a counterweight of a weight that is suitable for balancing the weight of the reel-holding carriage.
The reel-holding carriage supports a reel of plastic stretch film and an unwinding and pre-stretch unit arranged for unwinding and stretching or elongating the film made of synthetic plastic material.
The unwinding and pre-stretch unit is provided with a pair of prestretching rollers comprising a slow and a fast roller, respectively, upstream and downstream of the movement of the film, to stretch and unwind the extendible film and one or more deviating rollers for deviating the film during unwinding.
The unwinding and pre-stretch unit is provided with an electric motor, for example an alternating-current, direct-current or brushless electric motor, which motor is also supported by the reel-holding carriage and is able to rotate one of the two prestretching rollers that act as a driving (master) roller, which roller is typically the fast roller that via a belt transmission unit or cog transmission unit drives the other prestretching roller that acts as a driven (slave) roller, which roller is typically the slow roller. In this way, between the fast roller and the slow roller, a fixed transmission ratio is established as a function of the prestretching or elongation that it is desired to obtain on the film. In use, the film in fact passes from the slow roller to the fast roller and owing to the difference in rotation speed between the latter, set by the aforesaid transmission ratio, the aforesaid film is subjected to a prestretching or elongation force. This enables the portion of film between the two prestretching rollers to be stretched and elongated before the later is wound on the products, both for using the available film as efficiently as possible and for changing the mechanical features of the material of the film as a function of the product to be wound.
As known, the prestretching force enables the thickness of the film to be reduced significantly (typically from approximately 25/23 μm to approximately 6/7 μm) so as to increase the length thereof proportionally to wrap a greater number of products.
The prestretching force to which to subject the film to obtain a given elongation percentage depends both on the initial thickness of the film and on the physical/mechanical features of the material, such as composition, quantity and distribution of possible impurities and internal irregularities. For this reason, films of the same material and the same thickness belonging to different reels often have to be subjected to different prestretching forces to obtain similar elongation percentages.
The prestretching force further enables the mechanical features of the film to be changed. The suitably stretched material of the latter can in fact change from elastic behavior, in which the film tends to recover the original dimension at the end of the stress, to plastic behavior, in which the film undergoes a permanent deformation and does not regain the initial dimension at the end of stress. In this latter case the film of synthetic plastic material acts as a flexible and unextendable element, the same as a rope or a belt, and can be used, for example, to wrap groups of unstable products that have to be maintained firmly bound together.
The electric motor that drives the prestretching driving roller can be supplied by an alternator, positioned on the reel-holding carriage, and provided with a sprocket that engages a rack positioned on a coaxial fixed ring and arranged outside the rotating ring.
In this way, when the rotating ring rotates, the sprocket is rotated by the fixed rack and generates the current that supplies the motor.
In other machines, the alternator can be provided with a pulley rotated by a fixed belt.
The belt is arranged for rotating the pulley when the rotating ring is rotated that supports the alternator, so as to generate the current that drives the motor.
Alternatively, the electric motor can be driven by batteries positioned on the rotating ring on the side opposite the reel-holding carriage.
Still alternatively, the electric motor can be driven by creeping contacts, positioned and operating at the external fixed ring.
The unwinding and pre-stretch unit further comprises a control device, associated with the reel-holding carriage, arranged for varying the rotation speed of the driving prestretching roller, and thus the film unwinding speed in accordance with the shape or cross section of the product to be wound and with the corresponding angular position between the latter and the reel-holding carriage. This enables the wrapping traction or tension force of the film around the product, the so-called “pull” to be maintained more or less constant, to prevent breakage thereof or to prevent a value that is not suitable and appropriate to the type of product to be wound. For example, a relatively fragile single product has to be wound with sufficient tension to maintain the film adhering to the product but not with such as to deform or break the latter. On the other hand, a group of undeformable products placed on a pallet will have to be wound at greater tension to confer stability and compactness on the packed group.
The control device generally comprises a so-called “dandy” or “guide” roll, mounted on an elastic support that is movable away from and towards the product to be wound, as a function of the force exerted thereupon by the film wound around the product during a wrapping phase.
In this way, when the dandy roll moves towards or away from the product to be wrapped an electric signal is sent to a management and control unit, which, through the electric motor increases or decreases the rotation speed of the drive motor, and thus via the transmission unit the rotation speed of the driven roller, so as to increase or decrease the unwinding speed of the film, at the same rotation speed as the rotating ring and maintain more or less constant the prestretching force and the tension of the film.
When it is desired to wrap a product with an extendible film made of synthetic plastic material, the product is first positioned substantially at the vertical wrapping axis, and the wrapping machine is driven that moves the supporting structure.
The latter moves the reel-holding carriage along a circular or helical trajectory so as to wrap the products with several coils of film along the vertical wrapping axis, the latter substantially coinciding with the vertical axis of the products to be wound.
The aforesaid description, albeit with certain different technical details, can also extend to wrapping machines in which the supporting structure develops along a vertical plane and the products advance along a horizontal plane passing through the rotating ring to be wound by the film in successive coils along a horizontal wrapping axis.
A drawback of the aforesaid machines is the considerable weight of the rotating ring that during operation generates considerable forces of inertia that are mainly due to the weight of the electric motor, of the counterweights, of the dandy roll and, where present, of the alternator and of the batteries.
This greatly reduces the rotation speed of the rotating ring and consequently limits the productivity of the wrapping machine.
Further, to counteract these inertia forces it is necessary to stiffen significantly the supporting structure and the frame, with a consequent further increase in weight and costs.
A further drawback relates to the creeping contacts used to supply the electric motor, which on the one hand are subject to serious wear and thus have to be replaced frequently and on the other hand further limit the rotation speed of the rotating ring and therefore the productivity of the wrapping machine.
These contacts may further cause sparks and prevent the wrapping machine being installed in environments having a high level of humidity.
Further, where batteries are used, the latter, in addition to being costly, have to be recharged during machine downtime.
If an alternator is used, this causes an increase in the weight to be rotated and further generates current only after the rotating ring starts to rotate, which does not enable the film to be prestretched in an initial wrapping phase.
A further drawback of these machines lies in the operations that are necessary for varying the transmission ratio between the prestretching rollers to vary the prestretching or elongation of the film when it is desired to use different film made of synthetic plastic material, or when it is desired to wrap products of different types, for example groups of stacked products.
These operations, which comprise stopping the wrapping machine, dismantling the transmission unit and refitting a new transmission unit, are extremely slow and laborious and require specialized labor for the performance thereof.
For this reason, the known wrapping machines do not enable the film of synthetic plastic material of each reel to be used in an optimal manner, adjusting suitably the prestretching force in function of the physical and mechanical features of the film of the reel in use.
Still another drawback of the aforesaid wrapping machines consists of the difficulty of maintaining constant the tension of the film wound around the product, especially if the latter has a complex profile or shape, for example an elongated shape, and/or the machine has a rotating ring with high rotation speeds.
This is due to the fact that the reel-holding carriage travels a certain angular sector between the moment in which the dandy roll is affected by the variation in tension exerted by the film and the moment in which the management and control unit commands the electric motor that varies the rotation speed of the prestretching rollers.
This causes a delay in the dispatch of the electric control signal to the motor, a delay that is greater the greater the rotation speed of the rotating ring and/or the dimensions and the shape of the product to be wound. This delay in the feedback of the prestretching rollers may cause excessive tensioning of the film in non-desired portions of the product to be wound and may lead to the breaking of the film.